We studied the dynamics of left ventricular outflow tract gradients in hypertrophic cardiomyopathy. In 25 patients, we performed catherization with micromanometer catheters and a nonimaging scintillation probe to construct high temporal resolution left ventricular pressure-volume loops. In 11 patients without outflow tract gradients, the contour or the ejection phase was concave downward. In the 14 patients with outflow tract gradients, however, an inflection point occurred during the ejection phase, corresponding to onset of the left ventricular outflow tract gradient. After this inflection point, left ventricular pressure increased markedly, and the difference between peak left ventricular pressure and this inflection point correlated with the magnitude of the outflow tract gradient. This point appeared early during ejection and the mean percentage of left ventricular stroke volume that occurred after this point and during the outflow tract gradient was 62%. These data indicate that the outflow tract gradient in hypertrophic cardiomyopathy cannot represent cavity obliteration after left ventricular emptying, as over half of left ventricular stroke volume is ejected during the outflow tract gradient. These data provide further evidence that the outflow tract gradient represents true impedence to left ventricular ejection.